This means that software is written on one Unix computer, can be easily moved to another computer. For the companies, there is no need to depend on single computer vendors any longer.
Ever heard the buzzphrase open systems? In many ways, this is a synonym for Unix. You are free to buy computer hardware and software and that will be compatible with your grand computing scheme.
Yes, there will always be problems when mixing different hardware types and architectures. But at least you're free to buy all your computer equipment from a single vendor who has you over a barrel with proprietary hardware and software.